The present invention relates generally to a heat resistant Al die cast material and, more particularly, to a heat resistant Al die cast material suited as a part of an internal combustion system, such as a piston.
Conventional heat resistant Al materials consist of elements like Si, Cu, Mg, Ni and Ti added to Al at concentration levels appropriate for abrasion resistance, seizure resistance, and thermal resistance. An important application for heat resistant Al materials is pistons, which are a part of internal combustion systems. xe2x80x9cAl alloy cast metalxe2x80x9d is standardized in JIS H 5202 (1992). Table 1 in this standard lists the types of alloys and their codes, Table 2 lists chemical compositions, and Table 3 lists mechanical properties of cast metal test samples. Table 1 through Table 3 below summarize the JIS Table 1 through Table 3.
As shown in the right-hand column in Table 1, under the xe2x80x9cApplicationsxe2x80x9d header, the AC8A, AC8B and AC8C Al alloy die cast metals are used for pistons in automobiles.
xe2x80x9cMetal moldsxe2x80x9d listed under the xe2x80x9cType of Moldxe2x80x9d in the third column of Table 1 represent regular metal casting.
Table 2 shows the chemical compositions of the AC8A, AC8B and AC8C Al alloy die cast materials. AC8A is an Alxe2x80x94Sixe2x80x94Cuxe2x80x94Nixe2x80x94Mg alloy containing 0.8% to 1.3% Cu, 11.0% to 13.0% Si, 0.7% to 1.3% Mg, and 0.8% to 1.5% Ni. AC8B is an Alxe2x80x94Sixe2x80x94Cuxe2x80x94Nixe2x80x94Mg alloy containing 2.0% to 4.0% Cu, 8.5% to 10.5% Si, 0.5% to 1.5% Mg, and 0.1% to 1.0% Ni. AC8C is an Alxe2x80x94Sixe2x80x94Cuxe2x80x94Nixe2x80x94Mg alloy containing 2.0% to 4.0% Cu, 8.5% to 10.5% Si, 0.5% to 1.5% Mg and 0.5% to 1.5% Ni.
As shown in Table 2, Zn content is less than or equal to 0.15% in AC8A and less than or equal to 0.50% in AC8B and AB8C. xe2x80x9cLess than or equal toxe2x80x9d means that Zn content can be 0%. In other words, Zn content should not exceed the prescribed amount (0.15% or 0.5%).
Table 3 lists the mechanical properties of die cast test samples and provides information on whether or not any treatment is applied, and, if so, what type of treatment. For example, the xe2x80x9cFxe2x80x9d suffix that comes after the AC8A, AC8B and AC8C codes indicates that the alloy has only gone through a casting process. A xe2x80x9cT5xe2x80x9d suffix indicates that the alloy has been age hardened. A xe2x80x9cT6xe2x80x9d suffix indicates that the alloy has been age hardened after a solution treatment. For example, the AC8C-T6 alloy in the lower-most row goes through a solution treatment for approximately four hours at approximately 510xc2x0 C., followed by approximately 10 hours of age hardening at approximately 170xc2x0 C. The third column on Table 3 lists the tensile strengths. Tensile strength is lower for xe2x80x9cFxe2x80x9d compared with xe2x80x9cT5xe2x80x9d, while tensile strength is higher for xe2x80x9cT6xe2x80x9d compared with xe2x80x9cT5xe2x80x9d. Therefore, xe2x80x9cT5xe2x80x9d or xe2x80x9cT6xe2x80x9d treatment may be used for enhancing strength. These treatments are also effective for improving the dimensional stability during annealing.
Table 4 is a Reference Table 1 found in JIS H 5302 (1990). ADC10 and ADC12 are both Alxe2x80x94Sixe2x80x94Cu alloys, which do not contain Mg. Their compositions are given in JIS H 5302 (1990) and will not be listed here. ADC10 and ADC12 are Al alloy die cast metals whose compositions are different from the AC8A, AC8B and AC8C metals discussed above.
ADC10, which is an as-cast metal, has a tensile strength of 245 N/mm2, as shown in the third column of Table 4. ADC10 has a different composition and a much greater tensile strength than the AC8A-F, AC8B-F and AC8C-F metals mentioned above, whose tensile strengths are greater than or equal to 170 N/mm2. ADC12 exhibits similar properties.
While regular cast metals are produced by gravity casting, die cast metals are manufactured by high pressure casting. High pressure casting results in a more dense casting structure, which also results in higher strength.
The inventors of this invention assumed that it would be possible to achieve a much higher strength by treating die cast metals, if xe2x80x9cT5xe2x80x9d age hardening on the AC8A alloy increases the tensile strength from 170 N/mm2, to 190 N/mm2, and xe2x80x9cT6xe2x80x9d solvent treatment, followed by age hardening, increases AC8A""s tensile strength from 170 N/mm2 to 270 N/mm2.
The inventors first performed an experiment in which an AC8A die cast metal was manufactured and treated with T6 solution treatment, followed by age hardening.
The resulting AC8A-T6 metal was covered by blisters and unusable. It is believed that the alloy incorporates air and other gases during the casting process and remain in the die cast metal as bubbles. These bubbles expand under 510xc2x0 C. of heat during solvent treatment and lifted the Al alloy, which was softened under high heat.
Annealing temperature for the T5 age hardening, on the other hand, is around 200xc2x0 C. Nevertheless, even a die cast AC8A-T5 metal shows blistering to a lesser degree. This experiment has confirmed that the ADC compositions are made different from the AC compositions in the JIS in order to avoid this phenomenon.
The inventors of this invention, however, believed it would be possible to perform the T5 age hardening on die cast metals with AC compositions by modifying the AC compositions. As a result of various research projects, the inventors discovered compositions that would make the AC die cast metal amenable to the T5 treatment.
This invention provides heat resistant Al die cast material that contains 12.5% to 14.0% of Si, 3.0% to 4.5% of Cu, 1.4% to 2.0% of Mg, and 1.12% to 2.4% of Zn. This die cast material is age hardened after die casting.
Because the die cast material having the above composition is amenable to age hardening, the material offers a much higher mechanical strength and seizure resistance. When Zn content is less than 1.12%, the die cast metal is prone to anneal cracks. When Zn content is more than 2.4%, the material exhibits less toughness. Therefore, Zn content should preferably be 1.12% to 2.4%.
Appropriate amounts of Mg and Zn added to an Alxe2x80x94Sixe2x80x94Cu alloy has resulted in a die cast metal that is amenable to annealing. This type of alloy has not been previously commercialized because the material was too susceptible to anneal cracksxe2x80x94an important consideration for a die cast alloy.
For example, a thick cast metal having the ADC14 xe2x80x9cdie cast Al alloyxe2x80x9d composition (16.0% to 18.0% Si, 4.0% to 5.0% Cu, and 0.45% to 0.65% Mg), defined in JIS H 5302 (1990), tends to show many micro-cracks after casting.
Similarly, an alloy with 14.0% Si, 3.3% Cu, and 1.4% Mg contents also exhibits micro-cracks after casting.
This problem is caused by a reduced eutectic temperature, as low as 536xc2x0 C., depending on Cu and Mg contents. Because the eutectic temperature is lower, compressive stress concentrates where thick and thin parts of the die cast metal meet with each other before the annealed material becomes strong enough, as the molten metal in the metal cast in the shape of the end product solidifies and shrinks. As a result, the metal exhibits anneal cracks.
Zn has been added in an effort to prevent these micro-cracks. As a result, it was discovered that the eutectic temperature would go up to 547 to 554xc2x0 C., if equal amounts of Mg and Zn are added to Al at the same time as other elements. Further studies revealed that similar effects would be achieved as long as Zn concentration was 80% to 120% of the Mg content.